Vaccines are biological preparations that improve immunity to a particular disease. They are frequently used in the prophylaxis of humans and animals to protect against infectious diseases caused by bacteria, viruses and parasitic organisms. Therapeutic vaccines are also under investigation, such as for the treatment of cancer.
The antigens used in vaccines may include a variety of agents, such as killed pathogenic organisms, pathogenic organisms which are alive but modified or attenuated, proteins, recombinant proteins or fragments thereof. It is also often necessary to add an adjuvant to enhance the host immune response to the antigen, and in some cases slow the release of the antigens from the injection site.
A wide range of adjuvants have been studied for use in vaccines, including lipids and liposomes, in which an antigen of interest can be encapsulated within a lipid vesicle.
Glycolipids
Glycolipids are of interest as adjuvant ingredients as they can target specific receptors on antigen presenting cells (APC's). However, since most glycolipids are uncharged, a stable bilayer does not form when attempts are made to prepare glycolipid-liposome based vaccine carriers. According to present knowledge, a liposome or archaeosome composed solely of glycolipid(s) would not form a stable structure. This can be solved by adding phospholipids with associated charge to the glycolipid formulation.
For instance, archaeol has been isolated from hydrolysed polar lipid extracts of Halobacterium salinarum to use as the lipid precursor to chemically synthesize various polar lipids, including glycolipids (Sprott, G. D., Dicaire, C. J., Cote, J. P., and Whitfield, D. M. 2008. Glycobiology 18:559-565; Whitfield, D. M., Yu, S. H., Dicaire, C. J., and Sprott, G. D. 2010. Carbohydr. Res. 345:214-229). The lipids so generated are described as synthetic, or more precisely as semi-synthetic, because the lipid moiety with specific archaeal stereochemistry is of biological origin, whereas the polar head group is synthesized or conjugated to the free sn-1 hydroxyl of the glycerol backbone of the archaeol to give a new lipid structure. These glycolipids were mixed with phospholipids to make archaeosomes having a negative-charge, and with adjuvant activities that varied with the structure of the polar head group of the lipid (Sprott, G. D., Dicaire, C. J., Cote, J. P., and Whitfield, D. M. 2008. Glycobiology 18:559-565).
There are, however, several potential limitations with adding additional phospholipid as part of a glycolipid-liposome/archaeosome adjuvant. For instance, more lipids are required in the formulation, adding to complexity. In addition, the active glycolipid is diluted to much less than 100%, which can lead to reduced efficacy. Costs associated with synthesis also escalate as more lipids are required in the formulation. The stability of phosphodiester linkages to enzymatic and chemical attack is also not satisfactory, especially for harsh routes of delivery (e.g. oral), and any instability of the lipid vesicles resulting from these phosphodiester linkages can result in loss of cargo and therefore reduced efficacy.
Sulfated Glycolipids
Sulfated glycolipids (S-glycolipids) are found in some Halobacteria (Kates, M. 1996. J. Microbiol. Methods 25:113-128) and have been part of the total polar lipids (TPL) from several archaeal lipid extracts used to make archaeosomes. These archaeosomes had no improved adjuvant activity (Sprott, G. D., Sad, S., Fleming, L. P., Dicaire, C. J., Patel, G. B., and Krishnan, L. 2003. Archaea 1:151-164) or stability (Mathai, J. C., Sprott, G. D., and Zeidel, M. L. 2001. J. Biol. Chem. 276:27266-27271) compared to total polar lipid archaeosomes lacking S-glycolipids. Indeed the opposite was true, indicating that 5-glycolipid would not be the active ingredient. CD8+ T cell activity with total polar lipids (TPL) from extreme halophiles with S-glycolipid was relatively short-lasting compared to TPL of M. smithii or Thermoplasma acidophilum (Krishnan, L. and Sprott, G. D. 2003. Journal of Drug Targeting 11:515-524) that have no S-glycolipids. An improved antibody response with certain TPL of extreme halophiles was shown to be the result of the presence of a major lipid PGP—O—CH3, specifically archaetidylglycerolmethylphosphate (Whitfield, D. M., Yu, S. H., Dicaire, C. J., and Sprott, G. D. 2010. Carbohydr. Res. 345:214-229), rather than S-glycolipids. Other non-isoprenoid S-glycolipids such as sulfatides (predominantly 3-sulfate-β-D-Galp(1,1)Ceramide) (Patel, O., Pellicci, D. G., Gras, S., Sandoval-Romero, M. L., Uldrich, A. P., Mallevaey, T., Clarke, A. J., Le Nours, J., Theodossis, A., Carden, S. L., Gapin, L., Godfrey, D. I., Rossjohn, J. 2012. Nat. Immunol. 857-63) and the sulfolipid-1 (Geerdink, D.; Minnaard, A. J. 2014. Chem. Commun. 50:2286-2288) from mycobacteria (6-sulfate-α-D-Glcp(1,1)-α-D-Glcp substituted with 1 or more mycolic acids, typically 4) have been described to have immunological activity but with immunological activities distinct from archaeosomes.